Digital discriminators for FM signals of constant pulse width are widely used in linear systems. Problems typically encountered in such devices are that the bandwidth and center frequency are determined by tuned stages preceding the discriminator and the output voltages may be nonzero for zero frequency error. All of the above may be unstable with temperature.
Other types of frequency discriminators such as FOSTER-SEELEY, or ratio detectors, depend upon passband filters and circuit "Q" to determine both center frequency and the bandwidth. Both passband filters and circuit "Q" are temperature sensitive and lead to transfer function changes in the center frequency, dc output shifts, and slope changes in the df/dt vs. discriminator output curves.
In applications such as phase lock loop systems which have wide frequency ranges, the limited range of conventional phase detectors, .+-..pi. radians, require prepositioning if the starting point is more than 2.pi. radians away from the frequency of the incoming signal. The prepositioning for the voltage control oscillator transfer function is nonlinear and, therefore, a potential problem.
Accordingly, there is a need for a frequency discriminator having high linearity and high stability in which transfer functions are stable with respect to temperature and frequency. There is likewise a need for a phase and frequency detector having an expanded range of operation.